Nowadays, the stopping power (“SPR” for “Stopping Power Ratio”) of protons for the treatment planning of proton therapy may be obtained from single energy data of a computed tomography scan. The term “single energy” relates to the manner of computed tomography scan with only a single recording energy. So-called “CT values” are determined for the individual voxels from the reconstructed computed tomography scan or its raw data. These CT values are then assigned SPR values, that is to say, values which reproduce the stopping power. The nature of the CT values, which are also referred to hereinafter as “CT numbers”, is known to the person skilled in the art in this field. The CT numbers may be in Hounsfield units H.
The assignment of SPR values to CT numbers may be achieved by a look-up table or “Hounsfield look-up table” (HLUT). Based on calibration measurements, such a look-up table is generated for single energy data which assigns a CT value a discrete SPR value.
Such calibration measurements are disadvantageous as they imply an additional expense and an additional potential source of error.
It is common practice, depending on the treatment, to generate different look-up tables, e.g., one for treatments of the head and one for treatments of the body. An exemplary look-up table is shown in FIG. 1.
Recently, the use of dual energy CT images (“DECT”) was discovered as an alternative to the previous approach. The term “dual energy” likewise refers to the manner of the recording principle with two different recording energies. According to this principle, the spectral information of the dual energy data is used to determine the stopping power SPR with which SPR values may be directly calculated from CT numbers (see, for example, “Ion Range estimation by using dual energy computed tomography”; N. Hünemohr et. al.; Z. Med. Phys. 23 (2013) pp. 300-313 or “Experimental verification of ion stopping power prediction from dual energy CT data in tissue surrogates”; N. Hünemohr et. al.; Phys. Med. Biol. 59 (2014) pp. 83-96).
There are dual energy CT systems on the market which record the images with high and low energy with two different fields of view (“FOV”). The consequence of this is a spatial limitation of the prediction for the SPR as dual energy data is only possible within the smaller field of view (FOV).
This spatial limitation is disadvantageous for the treatment planning of proton therapy as calibration data is scanned again for those regions for which only single energy data is available, resulting in the aforementioned disadvantages.